This invention relates to spectrophotometers, and more particularly, to a method and apparatus for controlling measurements in a spectrophotometer of the type wherein monochromatic radiation of varying wavelengths is alternately directed to a reference cell and a sample cell containing a sample to be analyzed so as to form reference and sample beams which are received by a radiation detector which in turn, produces corresponding electrical outputs; an output of the detector responding to the sample beam is compared with that of the detector responding to the reference beam, and the difference between these outputs is derived as the transmittance of the sample.
Among such conventional spectrophotometers, are known spectrophotometers of so-called dynode feedback system using a photodetector in the form of a photomultiplier whose gain is automatically controlled such that outputs of the multiplier which responds to radiation transmitted through a reference cell may be constant at all wavelengths at which measurements are made. One example of these prior art spectrophotometers is shown in FIG. 6.
Referring to FIG. 6, there is illustrated at 1 a main section of a prior art spectrophotometer which includes a radiation source 2 capable of emitting monochromatic radiation of varying wavelengths, for example, a monochromator, a sample chamber or cell 3 containing a sample to be analyzed, a reference chamber or cell 4 to be described later, a photo detector in the form of a photomultiplier 5, and beam path switching means 6 for causing monochromatic radiation from the source 2 to alternately enter the sample cell 3 and the reference cell 4 to form sample and reference beams and directing in synchronism the sample and reference beams from the sample and reference cells 3 and 4 alternately to the photomultiplier 5. The reference cell 4 is used in the state that its transmittance is substantially 100% and it shows no characteristic wavelength response, that is, in an empty state (an empty cell is placed in the beam path) or in the state that the cell is charged with a standard material having a flat wavelength response and a high transparency. The beam path switching means 6 includes an inlet beam path switching device 7 called a sector adapted to be rotated by means of a motor (not shown) so as to alternately switch the radiation from the source 2 to the sample cell 3 and the reference cell 4 so as to form sample and reference beams, and an outlet beam path switching device 8 adapted to be rotated in synchronism with the inlet beam path switching device 7 so as to alternately direct the sample and reference beams to the photomultiplier 5. The beam paths extending from the inlet beam path switching device 7 to the outlet beam path switching device 8 through the sample and reference cells 3 and 4 are referred to as "sample path" and "reference path", respectively, in this specification. An output of the photomultiplier 5 is supplied to a sample/hold circuit 9 and an error control circuit 10 through an amplifier 11 as will be described in more detail.
The photomultiplier 5 or the amplifier 11 produces output signals S as shown in FIG. 7(A). In FIG. 7(A), a represents an impulse corresponding to the reference beam, i.e. -the beam transmitted through the reference cell 4, and b represents an impulse corresponding to the sample beam, i.e -the beam transmitted through the sample cell 3. A low level portion c between these impulses a and b corresponds to background radiation during the beam path switching and including dark current. The sample/hold circuit 9 is designed to effect sampling in synchronism with a timing pulse TA developed in the duration when the beam path switching means 6 is switched to provide the sample path, that is, the duration of an impulse b as shown in FIG. 7(B). The sample/hold circuit 9 thus produces an output corresponding to the level of an impulse b among output signals S of the amplifier 11, that is, an output corresponding to the intensity of the same beam. Further, the error control circuit 10 functions to derive a signal corresponding to the intensity of the reference beam among output signals S of the amplifier 11, compare it with a reference voltage to determine the difference between them, and control the sensitivity of the photomultiplier 5 in accordance with said difference in a feedback manner such that the impulses a representative of the reference beam intensity among output signals S of the amplifier 11 may be kept at a constant level. In the illustrated example, the error control circuit 10 consists of a circuit 10A for generating a reference voltage and a synchronization error integrator 10B adapted to operate in synchronism with a timing pulse TB developed in the duration when the beam path switching means 6 is switched to provide the reference path, that is, the duration of an impulse a as shown in Fig.7(C), for reading out the level of the impulse a and integrating the difference between said level and the reference voltage. Since the synchronization error integrator 10B is electrically connected to a high voltage source 12 which drives the photomultiplier 5, the output voltage of the source 12 is controlled by the output of the integrator 10B.
Since the detection system of the spectrophotometer shown in FIG. 6 is controlled such that impulses a among output signals S of the amplifier 11, that is, outputs of the detector which responds to the reference beam are kept at a constant level at all wavelengths, the output of the sample/hold circuit 9 not only corresponds to the intensity of the sample beam, but also directly represents the ratio of the intensity of the sample beam to the intensity of the reference beam at any wavelength, that is, the transmittance of the sample material itself at any wavelength.
In the above-mentioned feedback control system, in order that an output of the sample/hold circuit 9 precisely represents the transmittance of a sample itself at any wavelength, it must be satisfied that the sample and reference paths in the spectrophotometer main section have precisely identical wavelength response. However, even when the sample and reference paths are made identical in wavelength response in the stage of design and fabrication, they tend to show a difference in wavelength response due to staining and fogging of mirrors in both the beam paths as measurements are repeated. In such a case, an output of the sample/hold circuit 9 cannot accurately represent the transmittance of a sample itself at some wavelengths at which the sample and reference paths differ in wavelength response. Differently stated, in such a case, when scanning is carried out at each wavelength with the sample cell 3 emptied or set to 100% transmittance, outputs of the sample/hold circuit 9 show fluctuations with respect to the given level. Then, when measurement is made at each wavelength with the sample cell 3 charged with a sample material, an output of the sample/hold circuit 9 will not correctly represent the transmittance of the sample material.
One approach to solve this problem is that the operator manually operates a voltage regulator or other control such that outputs of the sample/hold circuit 9 may become constant when scanning is made at varying wavelengths with the sample cell 3 emptied or set to 100% transmittance. In fact, such manual adjustment is impractical because of complicatedness and inaccuracy.
It is, therefore, an object of the present invention to provide a method and apparatus for automatically controlling measurement in a spectrophotometer such that even when the sample and reference paths are not precisely identical in spectral response, an output of a detector which responds to a sample beam may correctly represent the transmittance of the sample at any wavelength.